An internal combustion engine of a motor vehicle may include a turbocharger to increase the pressure of the air entering the engine in order to improve combustion efficiency and torque output. The turbocharger may include a turbine disposed in an exhaust passage of the engine and a compressor disposed in an intake passage of the engine, the compressor driven by rotation of the turbine.
It is a problem with such turbocharger boosted engines that even if a variable geometry turbocharger is used the overall performance of the engine is compromised by the fact that the turbocharger has normally been designed to accommodate the maximum flow of exhaust gas from the engine to the turbine of the turbocharger when the engine is operating at maximum engine speed with a wide open throttle.
This requires the use of a turbocharger having large capacity in order to prevent unduly high backpressure on the engine and/or overspeeding of the turbocharger at very high exhaust gas flow rates. However, the use of a large capacity turbocharger has the disadvantage that at low engine speeds the flow rate of exhaust gas from the engine is too low to provide a sudden increase in boost pressure if there is a sudden increase in driver demand thereby resulting in what is often referred to as ‘turbo lag’.
In order to overcome this problem it has been proposed to use an electrically powered compressor to continuously fill in for the turbocharger at low to moderate engine speeds with the turbocharger taking over at higher engine speeds.
Although such an approach overcomes the aforesaid ‘turbo lag’ problem it introduces a further problem in that such continued use of the electrically powered compressor places a large drain on the electrical generation circuit of the motor vehicle, thereby increasing fuel consumption and in some cases requiring additional electrical storage capacity to be provided to deal with operation of the electrically powered compressor at very low engine speeds where the output from the electrical generation system of the motor vehicle is unable to cope with all of the electrical demands placed upon it.
It has further been proposed in JP-A-2010048225 to use first and second turbochargers and use an electric motor to spin up the second turbocharger so as to reduce fluctuations in turbocharging pressure when switching to the second turbocharger that would otherwise occur. However, such an arrangement is very complicated in construction requiring numerous valves and also requires the use of an electric motor that is able to withstand the very high temperatures associated with the turbine part of the second turbocharger.
It is an object of this disclosure to provide a boosted engine system that overcomes the problems associated with the aforesaid prior art. In one example, the issues described above may be addressed by a method, comprising: responsive to a demanded torque being above a threshold while providing boosted air at a first pressure to an engine via a turbocharger compressor, operating an electric compressor arranged in parallel with the turbocharger compressor and fluidly disconnected from the engine to increase a second pressure in a recirculation circuit around the electric compressor; and responsive to the second pressure reaching the first pressure, fluidly connecting the electric compressor to the engine. In this way, a torque demand of the engine may be met without continuously operating the electric compressor and increasing an electrical demand of the engine. Further, by initially fluidly disconnecting the electric compressor from the engine while the pressure (boost pressure) output by the electric compressor is built up (increased) and only fluidly connecting the electric compressor and the engine upon reaching the pressure of boosted air provided to the engine by the turbocharger compressor, a sudden drop in boost pressure upon operating the electric compressor and connecting the electric compressor to the engine is avoided. As a result, increased boost pressure to meet the operator torque demand may be provided more efficiently and quickly, thereby increasing engine performance and vehicle operator satisfaction.
In another example, the issues described above may be addressed by a boosted engine system for a motor vehicle comprising an internal combustion engine, a small capacity variable geometry turbocharger sized to meet demands for boosted air up to a predefined supply level and an electrically powered compressor arranged in parallel to the turbocharger, the engine being arranged to receive a primary supply of boosted air during normal engine running below the predefined supply level from the turbocharger and to receive a supplementary supply of boosted air from the electrically powered compressor when it is required to meet a temporary high demand for boosted air above the predefined supply level and a recirculation circuit to build up pressure when the electrically powered compressor is started before the electrically powered compressor is connected to the engine, the recirculation circuit comprising an electrically controlled recirculation valve to control the flow of air through a conduit linking an outlet from the electrically powered compressor to an inlet to the electrically powered compressor, an electrically controlled shut-off valve located between the outlet from the electrically powered compressor and the engine to selectively isolate the outlet from the electrically powered compressor from the engine and a backflow control valve to prevent the backflow of air from the inlet to the electrically powered compressor to atmosphere wherein upon starting of the electrically powered compressor the electrically controlled recirculation valve is kept open and the electrically controlled shut-off valve is kept closed until the pressure in the recirculation circuit has reached a predefined limit whereupon the electrically controlled shut-off valve is opened and the electrically controlled recirculation valve is closed.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.